The Blood-Aqueous Barrier (“BAB”) is a metabolic or cellular structure in the anterior chamber of the eye (“ACE”) that restricts the passage of various chemical substances and microscopic objects, such as bacteria, between the bloodstream and the ACE tissue itself, while still allowing the passage of substances essential to metabolic function, such as oxygen, sodium and potassium salts and glucose. During duress, which may include traumatic injury, radiation exposure or diseases, such as cancer, the normal function of the BAB is adversely affected and results in a range of typically locally blocked analytes crossing into the anterior chamber of the eye. As such, detection of such analytes in the human eye can be a reliable indicator of a person's health.
In addition to monitoring the analytes that cross the BAB during injury or disease, the monitoring of analytes that cross the BAB during normal conditions may also indicate the health condition of a subject. For example, a sufficiently hydrated person typically has a specific concentration range of sodium salts (natremia), and the ability to detect raised or lowered concentrations of these sodium salts may provide a reliable indication of a person's hydration level. On the one hand, some of the methods typically used for accurately determining the hydration status, such as Total Body Water (“TBW”) measurements and invasive measurements of plasma osmolality, are complex and require a clinical setting.
On the other hand, some of the more rapid and non-invasive techniques, such as bioimpedence measurements made through the skin using weak electrical currents, salivary osmolality measurements (for hydration), and monitoring physical signs of dehydration or diabetes, require baseline characterizations and may not be very specific. Additionally, non-invasive measurement modalities that require electromagnetic/optic probing made through the skin are easily corrupted by skin temperature, perspiration and body position of the subject, therefore limiting their application. Further, many of the non-invasive methods measure the concentration of analytes in the eye using laser assisted measurements of absorption at a select few wavelengths.